See-through-wall and Ground Penetrating Radars (GPR) have broad civilian and military applications in finding objects or explosives such as Improvised Explosive Devices (IEDs) inside or behind walls or buried under ground. The radar transmits impulses or continuous waveforms that can penetrate soil, concrete, glass or wood and captures the reflected signals. The time difference between transmission of a waveform and reception of the reflected signals (round-trip time) as well as amplitude and phase information are utilized to detect the target range and to identify the target from its cross section. In more advanced radars, a signal processor is employed to construct a 2-D or 3-D image of the scanned area using the reflected pulses from the target. For example, land-survey GPRs are mounted on a vehicle to scan the roads to explore the subsurface of the ground. An example GPR scan scenario is illustrated in FIG. 1a, which shows a GPR 100 scanning in the x direction at various scan points. A radar beam from GPR 100 propagates into the ground in the y direction, which contains a buried target 105. At every scan point GPR 100 emits radar pulses and records the received reflected pulses, which come from different depths to thereby allow an imaging of the ground. The GPR performs the appropriate signal processing on the returned pulses to transform the information into a two-dimensional (2-D) image as seen in FIG. 1b. The translation of the GPR in the x-direction (which is the scan speed) is typically on the order of a few miles per hour. To precisely map an area, the scan points should be as close as possible to each other, provided that the antenna beam is narrow. To construct the RF image, the radar antenna distance to the ground should be unchanged during the scan and the scan speed should also be constant. Any translation of the GPR in range with respect to the target (in this example, in the y-direction) will result in variations in round-trip time of transmitted pulses which translate to changes in detected depths of different points of an object.
FIG. 2a illustrates the errors introduced by a translation of GPR 100 in range (in this example, the y direction) with respect to the target as it scans. Although the target is unchanged from FIG. 1a, the resulting radar image of the ground shown in FIG. 2b indicates a concavity in the target (and the ground surface) corresponding to a mirror image of the y-direction translation of the radar. Such fluctuations in the height of the radar as it scans are inevitable if the radar is hand-held or transported by a helicopter or other type of aircraft. Moreover, even if the radar is mounted on a vehicle, it will have height variations as the suspension accommodates rocks and other uneven terrain.
Variations in the speed of the vehicle also affect the quality of the produced images by introducing uncertainty in the exact position of scan points along the scan path. More specifically, the constructed image and targets dimensions will be unevenly skewed or smeared responsive to the scan speed variations. To alleviate this problem, vehicle speed data are fed to the radar by a tachometer attached to a wheel or through an analog/digital interface to the vehicle's speedometer to calculate the relative position of each scan point. This method develops inaccuracies as the position error will accumulate through the scan. Moreover, radar scan speed is typically greater than the response time of conventional vehicle-mounted speedometers, especially tachometers such that the position information associated with every scan point may not be updated accordingly. The above mentioned problems are more serious for hand-held radars as the scanner motion is solely controlled by the operator's skills. Uneven speed of manually sliding the scanner over the target and operator's hand vibrations and unintentional drifts in the radar distance to the surface during the scan result in significant degradation in image quality.
Accordingly, there is a need in the art for radars which compensate for range translation as they scan. In addition, there is a need in the art for radars which compensate for scan speed variations.